The invention relates to a field winding type synchronous machine.
A field winding type synchronous machine is a synchronous machine, and some methods for starting it are known. A representative method uses an inverter. However, if a synchronous machine is not required to operate at a variable speed after being synchronously incorporated into an electrical power system, then the inverter will be needed only at start-up, and an initial cost due to the inverter and an expense, such as for installation thereof will become excessive.
In one of the starting methods which do not need an inverter, a direct on line method is used to start a synchronous machine (hereinafter, referred to as “DOL start-up”). This is the same starting method as a full voltage starting method by which an induction motor is started. In this method, although being a synchronous machine, it is started using characteristics of an induction motor only at start-up. At this time, to obtain the characteristics of an induction motor, a field winding on the rotor side is usually excited by a direct current from an AC brushless exciter (AC exciter), but the field winding is disconnected from the AC exciter only at start-up and brought into a short-circuit condition. Also, to reduce a starting current, a discharge resistor (hereinafter, referred to as “DR”) is put into the shorted circuit.
However, a DR is needed only at start-up and adversely provides a loss during a stationary operation at a synchronous speed, lowering efficiency of an electric motor. Thus, during a synchronized operation, the DR has to be disconnected. Also, to switch from DOL start-up to the synchronized operation, there is needed a process for switching to a direct current from an AC exciter when started and accelerated to about the synchronous speed.
In a field winding type synchronous machine, various circuits for switching from start-up to a synchronized operation have been studied. For example, a conventional field winding type synchronous machine discloses that a field winding is connected to an AC brushless exciter and a rectification circuit via a thyristor. Also, a DR is disposed in parallel to the field winding. To smooth a DC voltage from the rectification circuit, a smoothing capacitor is disposed in parallel to the rectification circuit.